Radiation sensors are used in a wide variety of fields. Particularly in the field of testing of materials and coatings for their out-of-doors stability and other stressful environments, it is desired to make use of accelerated aging tests under artificial conditions. However, these artificial conditions must be constantly correlated with natural conditions in order to assure the artificial testing conditions provide useful data.
In outdoor aging tests, as well as stations for accelerated aging, difficulties have been achieved in providing proper measurement of the total exposure of sunlight or other radiation received by the object being tested.
The most significant portion of solar energy is its ultraviolet component. Despite the importance of the monitoring of solar ultraviolet radiation applied to exposed specimens, such monitoring has been restricted by the available technology that limits the accuracy and reliability of radiometers intended for such use. As one example, low latitude testing stations, of course, can provide a great deal of sunlight exposure to test objects. The temperatures can become so hot, and the humidity can become so high, to shorten the useful life of current radiometers, and also to cause them to exhibit inaccuracies of readout.
In the prior art, an ultraviolet radiometer has been commercially used for outdoor testing purposes. As an example of current technology, a TUVR radiometer employing a wide band UV filter is available and advertised as covering a wide range of 295-380 nm. ultraviolet wavelength. These wave length limits are actually the cut-on and the cut-off wave lengths at which 5% transmittence through the apparatus filter is achieved. The half peak band width is about 60 nm. covering the approximate wave length interval 300-370 nm. A selenium barrier-layer photocell, the spectral sensitivity of which to shorter wave length UV is much less than its peak sensitivity to visible light, is used. This means that it is much more sensitive to fluctuations in the longer wave length UVA portions of the solar spectrum than to variations to the shorter wave length UVB region. The radiometer is not thermo-regulated, and unless the radiation filter used is designed for accuracy at elevated temperatures, there will be a slight shift in peak response to longer wave lengths as temperature increases.
The above described radiometer, and other radiometers, are also sensitive to moisture, which can condense at night after having seeped into the interior of the radiometer during the daytime in high-humidity areas. The condensed droplets can provide significant inaccuracy to subsequent radiation sensing, and can also accelerate deterioration of the device. Similarly, high temperatures can damage both the interference light filters and photosensors used.
In accordance with this invention, a radiation detector is provided which is capable of withstanding the rigors of all-day exposure to the sun and weather in tropical areas, containing to operate and provide accurate radiation exposure data for periods of time which tend to greatly exceed the useful lives of previous photodetectors used for that purpose. Additionally, the radiation detector of this invention may be used in other harsh environments as well, and for any desired purpose.